Silicon nitride or silicon oxide layers may be made on silicon and on other substrates. These layers may perform as anti-reflecting, passivation, or electronically insulating layers. By design, the nitride and oxide layers are highly resistant to material diffusion and are also chemically resistant to most reactions. Such layers present an obstacle when it is desired to form direct contact with the substrate underneath them. In some cases, it is desired to selectively etch specific areas of these impervious layers as recited in a desired pattern while keeping the rest of the coating intact. Typical methods for patterned etching are laser ablation and chemical etching using a photoresist layer or protective mask. Laser ablation uses a focused laser combined with a tracking mirror to effectively remove the silicon nitride layers from the surface. Adjustment of the laser power and wavelength allows for selective removal of just the top nitride or oxide layers on top of the silicon. Chemical etching is another method for surface coating layer removal.
Consider an example of printing a pattern on nitride-coated silicon using metal-containing ink or paste with the intention of forming an electrical contact between the metal and the silicon underneath the nitride layer. One approach to achieving this goal is coating the substrate with a protective layer mask as recited in a predetermined pattern. This mask keeps the desired pattern exposed; then, the substrate is treated with etching reagents. One manner for etching these layers involves using wet reagents based on hydrofluoric acid (HF) and/or phosphoric acid (H3PO4). These reagents may be applied by involve dipping the substrate or covering it with a film of the liquid reagent. The protective layer can later be removed. After the desired pattern is etched, it is possible to print the metal ink on the now-exposed pattern. This approach involves three steps: the two steps of forming and removing of the protective layer, and the etching step itself. In addition, it poses technical requirements of aligning the printer as recited in the pattern. Another approach is adding the etching reagent directly into the metallic ink or paste, thus allowing the reagents in the ink to etch its way through the nitride or oxide layer. This approach eliminates the use of protective coatings and a dedicated etching step.
Currently, this objective may be achieved by adding low melting point glass frits to the printed media, e.g., addition of glass frit to a printed media of silver paste composed of silver particles. After printing the paste, the whole substrate is exposed to elevated temperatures sufficient to melt the glass frit. The molten glass frit reacts with the silicon nitride or oxide layer and allows the silver particles to diffuse and contact the silicon, layer underneath. The areas that were not printed with the paste are not etched. The glass frit may act as a flux that helps to etch the surface of metallic particles in the media and helps joining them together to increase adhesion.
The glass frit may be added in the form of powder to the printed media. This approach has drawbacks. The particle size of the powder may limit its application or print methods. Small particle size is required to allow the particles to pass through a small print nozzle. Typically, the particle diameter in the ink must be at least 20 times smaller than the inner diameter of the nozzle or print opening. More preferably, the particle size will be at least 50 times smaller than the nozzle diameter. In one example, inkjet printing nozzles may have a diameter of 20 microns. It is difficult to obtain glass frit materials below 1 micron in diameter, as the glass frit is manufactured by milling larger sizes of the glass composition into small particles. As a result, it is almost impossible to form inks that comprise stable dispersions of glass frits, and it is not trivial to inkjet such formulations.
Aspects of the present invention utilize in situ formation of glass and ceramics to assist a burnthrough of silicon nitride and silicon oxide layers using oxides, oxide forming precursors, or mixtures thereof. Embodiments of the present invention produce the glass frit in situ from soluble or dispersible components, which upon exposure to elevated temperatures form the desired glass that etches the nitride or oxide layer, enabling the metallic particles to reach the silicon substrate underneath the etched layer.
An advantage of this approach, as reflected from the previous discussion about the drawbacks of conventional glass frits, is that the glass composition can be determined by modifying the glass forming components and proportions. The glass forming components can be fully dissolved in the media, thus eliminating problems associated with dispersion. This approach provides a very flexible tool for designing glass with the optimal properties. This approach is valid even when the glass forming components are not soluble in the media.
Identity of the Active Species Responsible for Etching:
In either case, whether ready-made glass frit or glass-forming materials are employed, the nature of the active species is ambiguous (e.g., not well defined) due to other components that may be present in the formulations. These include metal oxides and salts. Such additives can react with the molten ready-made glass or with the glass-forming materials, thus creating a new species with different activity and chemical definition. The active species responsible for etching the nitride or oxide coating is a relatively low-melting inorganic material, but its identity may not be necessarily defined as glass. The next paragraphs further explain this point and show the broad definitions and interpretations available for glasses and ceramics.
Glasses are noncrystalline structures, usually consisting of mixtures of oxides, mainly of silicon, boron, phosphorus, potassium, sodium, lead, antimony, bismuth as well as other elements. It is also possible to have glasses that become crystalline at room temperature, and may not be defined as glasses under certain terms. Glasses may contain negatively charged elements other than oxygen, such as in the case of fluorosilicate and beryllium fluoride-based glasses.
Ceramics:
From the Kirk Othmer Encyclopedia of Chemical Technology: “Ceramics may be defined as a class of inorganic, nonmetallic solids that are subjected to high temperature in manufacture or use. Ceramics are distinguished both from metals and metallic alloys and from organic materials such as polymers and plastics, and although syntheses may involve solutions or the final products are solids. The most common ceramics are oxides, carbides (qv), and nitrides (qv), but suicides, borides, phosphides, sulfides, tellurides, and selenides are ceramics, as well as elemental materials such as carbon and silicon. Ceramic synthesis and processing generally involve high temperatures and the resulting materials are refractory or heat resistant. Ceramics are commonly thought to include only polycrystalline materials, but glasses, which are noncrystalline, and single-crystal materials such as ruby lasers, are classified as ceramics materials.” From the foregoing definition, glasses fall into the wider group of ceramic materials.
Definition of Active Etching Species as Recited in Embodiments of the Invention:
As previously noted, the active species responsible for etching are low melting inorganic materials; however, the identity of these inorganic species may not be known due to the complexity of the etching process, which include reactions of the formulation components between themselves, reactions with the nitride or oxide coating, as well as reactions with the substrate beneath the coating. From the previous discussion, it is seen that the definition of the active species responsible for the etching is ambiguous (e.g., glass or ceramic). Therefore, in this disclosure, the glass frits are described as low-melt ceramics, thus defining aspects of the present invention as low-melt ceramic precursors, rather than glass-forming components. This definition includes glasses, as well as low-melting point inorganic materials, which may not be defined as glasses.
Low-melt ceramic precursors may be oxides of boron, bismuth, phosphorus, antimony, arsenic, tin, lead, zinc, cerium, aluminum, thorium, indium, as well as other elements. Also included are compounds that decompose to give oxides, hydroxides, or salts upon treatment at elevated temperatures. Examples include organic derivatives where the element of interest is covalently connected to organic structures, such as in boronic acids, boronate esters, dialkyltin oxides, etc., or by coordinative bond, such as in zinc-EDTA complex, bismuth-salicylic acid complexes, bismuth acetylacetonate, etc. Inorganic salts such as beryllium fluoride having a melting point of 554° C. are included as well.
Another benefit that arises from using ready-made glass frits known in the art, or low-melt ceramic precursors as herein disclosed, is better adhesion of the coating to the substrate, since the molten ceramics, once cooled and solidified, function as a binder.
Another approach according to aspects of the present invention involves organic fluoride salts and fluorine-containing polymers to assist burnthrough of silicon nitride and silicon oxide layers. Certain phosphate and fluoride salts are capable of etching the nitride or oxide layers. U.S. Pat. No. 7,837,890 describes formulation of printing media paste using ammonium fluoride (NH4F). The rational behind using ammonium fluoride is that it can decompose to hydrogen fluoride upon treatment at elevated temperatures
The drawbacks of ammonium fluoride and also the analogues ammonium bifluoride (NH4FHF) salts is that they are soluble only in water, only slightly soluble in alcohol, and cannot dissolve in common organic solvents. In order to use them in organic based formulations, it is necessary to disperse them. This fact presents a serious obstacle in using these materials in low viscosity liquids, such as inks, and limits their use to pastes where the high viscosity assists in forming homogenous dispersions stable long enough for practical use.
Herein is disclosed, a fluoride derivative never before tested. The fluoride derivative is a quaternary ammonium fluoride salt. The material shown at the example is tetraethylammonium fluoride. This material is easily soluble in water as well as in common organic solvents. This material, when added to a mixture of nickel nanoparticles and applied on silicon nitride coated silicon, clearly showed capabilities to etch the nitride coating and form an electrical contact between the cured nickel film and the silicon substrate.